1. Field of the Invention
The present invention relates to a mechanical anchor to be used as a reaction force resisting unit in a loading test for pile or subsoil, in a cone penetration test in subsoil exploration, and in a press-in operation for pile, caisson, etc. In particular, the present invention relates to a so-called pre-boring type mechanical anchor reusable by recovering after the first use and to be placed into an anchor hole formed in advance.
2. Description of the Background Art
A recovery type conventional mechanical anchor used commonly in the past comprises a tension rod connected to a support mechanism installed on the ground, an anchor unit divided longitudinally into two parts and connected to the tension rod so that the divided anchor units can be moved in lateral direction, and a fluid pressure jack arranged on inner side of the anchor unit and used for opening or closing the anchor units. In this jack, a cylinder is disposed in lateral direction so that a piston rod is brought in or out in a direction perpendicular to inner surface of each of the divided anchor units. When the anchor unit is lifted up and suspended by the support mechanism, hydraulic pressure is supplied into the fluid pressure jack, and the piston rod is protruded to open the divided anchor units. The anchor units are brought into close contact with the subsoil, and resisting force against tensile force is increased. This prior art is disclosed, for instance, in Japanese Utility Model Publication Laid-Open 52-46013, Japanese Patent Publication 4-13496, Japanese Patent Publication Laid-Open 11-256574, and U.S. Pat. No. 6,210,077 B1.
However, in the conventional type mechanical anchor as described above, the fluid pressure jack is disposed in lateral direction, and it is not possible to design the fluid pressure jack with a length shorter than a predetermined value, and it is difficult to design the anchor unit with smaller diameter. This is an important problem because, in a pre-boring type mechanical anchor, which is inserted into an anchor hole formed in advance, the above design gives adverse effect on the diameter of the anchor hole.
For instance, in the screw type reaction pile as described in Japanese Utility Model Publication Laid-Open 52-46013, the cylinder is arranged between screw shafts longitudinally divided into two parts and in a direction perpendicular to the screw shafts. In this case, diameter of the screw shaft when the anchor units are opened is determined by the sum of the length of the cylinder and the stroke of the cylinder rod. Thus, it is not possible to manufacture a small caliber type reaction pile with diameter of 300 mm or less.
The present inventors have experimentally found follows: When a load is applied on the subsoil in lateral direction using the divided anchor units, the subsoil can be compressed and effective shearing section can be formed in a range with a central angle of 70xc2x0 to 80xc2x0. From these results, it can be understood that, if the number of the divided anchor units is preferably 3 or 4 rather than 2 because this makes it possible to increase the shearing sectional area and to induce higher anchoring force. However, in the conventional example with the jack arranged in lateral direction, it is practically impossible to design the divided anchor units in three or more divided parts because this requires more complicated structure in the connection between the divided anchor units and the jack.
The Japanese Patent Publication 4-13496 discloses a structure with the cylinder arranged in longitudinal direction. Specifically, one end of the cylinder is mounted on one of the divided anchor units, and an inclined surface in contact with the tip of the piston rod is provided inside the divided anchor units. In this case, a force in horizontal direction is applied on the mounting portion of one of the divided anchor units. As a result, the forces applied on the two divided anchor units are not equal to each other. This results in such a problem that the opening of the divided anchor units differs widely from each other.
To solve the above problems, it is an object of the present invention to provide a mechanical anchor, which is designed with small caliber and easy to handle, and which can be recovered and re-used and is suitable for the so-called pre-boring type with a boring hole formed in advance.
To attain the above object, the mechanical anchor of the present invention comprises an anchor unit in cylindrical shape connected to a tension rod, said anchor unit being equally divided longitudinally into 3 or 4 parts (divided anchor units), each of the divided anchor units is moved in parallel in lateral direction, and a driving member is provided in longitudinal direction to open or close the anchor units so that driving force is equally applied on the divided anchor units on upper and lower sides of the driving member.
As the driving member, it is preferable to use a fluid pressure cylinder such as hydraulic cylinder or air cylinder.
For the purpose of equally transmitting the driving force of the driving member, a mounting fixture having a mounting portion facing to each of the divided anchor units is provided on upper and lower ends of the driving member, and it is preferable to connect each mounting portion with each of the divided anchor units facing to it using a link arm, which is moved by the driving force of the driving member.